Ultra-pure hydrogen can be produced conveniently using metal membranes. Palladium alloy membranes are frequently used to purify hydrogen for compound semiconductor manufacturing, and in laboratories. It is also advantageous to use membrane reactors to perform chemical reactions more efficiently. Unfortunately, the high cost of palladium prevents metal membrane technology from replacing large-scale hydrogen purification methods such as pressure swing adsorption. Filters for hot gas cleanup have been fabricated using iron aluminide particles that resist oxidation and degradation by gaseous impurities commonly present in coal gas. Pouring the molten alloy through a nozzle and atomizing the stream with high-velocity gas such as helium produces spherical microparticles with a narrow size distribution. Alloys with extra aluminum such as Fe-16Al-2Cr (weight percent) can form a thermally grown aluminum oxide layer on the surface that protects the metal from further degradation. An advantage of thermally grown layers compared to extrinsic coatings is more uniform surface coverage with fewer defects. For example, chrome-containing alloys have been shown to form a continuous nitride layer at 1373 K that resists leaching in the acidic environment of a proton exchange membrane fuel cell (PEMFC).
Porous metal as a palladium membrane support has advantages of similar coefficient of thermal expansion to palladium, increased strength compared to porous ceramic or glass, and the potential to more easily seal into a module. Porous metal tubes and sheet are commercially available in pore sizes as small as 0.1 μm particle retention. However, the as-received surface roughness and pore size is too great to successfully deposit a thin (<10 μm), pinhole-free palladium or palladium alloy film. The deposition of a palladium film without defects depends on the ability to span all of the pores in the support membrane. Therefore, the critical qualities of a palladium membrane support are low surface roughness and small pore size. Metallic atoms from stainless steel such as chromium are also known to diffuse through into palladium and cause a decrease in membrane permeability. Metallic inter-diffusion that occurs between palladium and porous stainless steel at temperatures ≧450° C. has been reduced by thin layers of oxide, nitride, or refractory metal; membrane stability then depends on the stability of the intermediate layer.
Metals with bcc structure such as vanadium, niobium, tantalum, and titanium have higher hydrogen permeabilities than palladium but suffer from surface contamination and hydrogen embrittlement. However, alloys of these metals may possess some of the properties desired in a metal membrane including high hydrogen permeability, low cost, and ductility under hydrogen over a large temperature range. Generally, these metals require a thin film of palladium on their surfaces to catalyze the dissociation of molecular hydrogen and enable absorption into the metal (and recombination on the downstream side). The palladium film thickness required is generally much less than a freestanding or supported foil membrane. Palladium-coated vanadium, niobium, and tantalum membranes have been studied. These membranes embrittle due to the formation of hydride phases, especially at lower temperatures. Vanadium alloys that have been investigated for hydrogen membranes include V—Ni, V—Al, V—Ni—Al, and V—Cr—Ti. Promising ternary alloys based on the V—Ti—Ni or Ta—Ti—Ni systems exhibit hydrogen permeabilities comparable to palladium and they resist embrittlement. Besides embrittlement, another problem experienced by metal composite membranes is metallic inter-diffusion between the substrate foil and the palladium overcoat. For example, palladium-coated tantalum membranes have experienced a slow decline in hydrogen flux at temperatures ≧400° C.
Ceramic membranes are fabricated from metal oxide particles that are suspended in a solution or slurry, extruded, dried, and fired. Layers of finer particles may be applied using sol-gel dip coating or slip casting followed by firing. Smooth surfaces and very small pore sizes (<1 nm) can be obtained. However, ceramics are brittle, multiple coating and firing steps are labor intensive (expensive), and the coefficient of thermal expansion (CTE) can vary substantially from commonly used hydrogen separating membrane materials such as palladium. Metal oxides commonly used in membrane fabrication include alumina, titania, and zirconia. Porous ceramic layers can also be applied to porous metal supports using these methods, although the difference in CTE may cause such layers to crack and delaminate at elevated temperatures.
Other porous structures used to support thin films of hydrogen separating membrane materials include silica glass (VYCOR), perforated silicon, and anodic alumina. These membranes are formed by selectively etching away material to form pores. These materials are fragile, and a further disadvantage of silica is that it is attacked by steam.
The best commercially available metal membrane support materials have larger pores and rougher surfaces than the porous metal membrane material described in this invention. The ability to deposit a continuous, defect-free coating of hydrogen separating membrane material onto a porous support is directly related to the pore size and roughness of the support. Smaller pores and a smoother membrane support surface have been correlated with the ability to obtain essentially continuous hydrogen separating membrane films on the order of a few microns thick or less.
Since materials utilized for separating hydrogen rely primarily on costly palladium and palladium alloys, and considering that hydrogen flux is generally inversely proportional to membrane thickness, minimizing the thickness of the separating layer is highly desirable.